1. Field of the Invention
The present invention relates to a method of manufacturing an optical element. In particular, the invention relates to a method of manufacturing an optical element having an aspherical optical surface having a rotational symmetry.
2. Brief Description of Related Art
The optical element having the optical surface is, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, and systems used for imaging structures, such as structures formed on a mask or reticle, onto a radiation sensitive substrate, such as a resist, in a lithographic method. The success of such an optical system is substantially determined by the accuracy with which the optical surface can be machined or manufactured to have a target shape determined by a designer of the optical system. In such manufacture it is necessary to compare the shape of the machined optical surface with its target shape, and to determine differences between the machined and target surfaces. The optical surface may then be further machined at those portions where differences between the machined and target surfaces exceed e.g. predefined thresholds.
Interferometric apparatuses are commonly used for high precision measurements of optical surfaces. Examples of such apparatus are disclosed in U.S. Pat. No. 4,732,483, U.S. Pat. No. 4,340,306, U.S. Pat. No. 5,473,434, U.S. Pat. No. 5,777,741, U.S. Pat. No. 5,488,477. The entire contents of these documents are incorporated herein by reference.
The conventional interferometer apparatus for measuring a spherical optical surface typically includes a source of sufficiently coherent light and an interferometer optics for generating a beam of measuring light incident on the surface to be tested, such that wave fronts of the measuring light have, at a position of the surface to be tested, a same shape as the target shape of the surface under test. In such a situation, the beam of measuring light is orthogonally incident on the surface under test, and is reflected therefrom to travel back towards the interferometer optics. Thereafter, the light of the measuring beam reflected from the surface under test is superimposed with light reflected from a reference surface and deviations of the shape of the surface under test and its target shape are determined from a resulting interference pattern.
While spherical wave fronts for testing spherical optical surfaces may be generated with a relatively high precision by conventional interferometer optics, more advanced optics, which are also referred to as compensators, null lens arrangements, or K-systems, are necessary to generate beams of measuring light having aspherical wave fronts such that the light is orthogonally incident at each location of the aspherical surface under test. Background information relating to null lens arrangements or compensators is available e.g. from the text book of Daniel Malacara “Optical Shop Testing”, 2nd Edition, John Wiley & Sons, Inc. 1992, Chapter 12.
For many types of aspherical optical surfaces to be tested it is necessary to provide a null lens system or a compensator having one or more lenses with a diameter which corresponds to a diameter of the aspherical surface under test. In particular, for aspherical surfaces having a convex shape, the diameters of lenses of the compensator may have to be greater than the diameter of the aspherical lens.
Manufacture of null lens systems having lenses with a great diameter and having a high accuracy is a considerable problem and not only incurs high costs. From the article by M. Bray, “Stitching interferometer for large optics: Recent Developments of a System for Laser Megajoule Components”, Lawrence Livermore Nat. Lab., CEA. in Proc. SPIE—Int. Soc. Opt. Eng. (USA), USA: SPIE—Int. Soc. Opt. Eng., vol. 3492, pt. 1-2[+suppl.], 1999, pages 946-956, there is known a method of testing a large mirror having a spherical shape by performing interferometric tests at a plurality of overlapping portions or sub-apertures of the optical surface to be tested. Each portion has a lower diameter than the surface to be tested. Measured surface data of each portion are then stitched together to generate surface data representing a map of the surface shape of the tested mirror. The data processing for stitching the various data portions includes determining magnitudes of a piston term, a tilt, and a lateral translation between adjacent overlapping portions. This is possible since the mirror has a spherical shape such that each measured portion of the whole surface represents a small portion of the overall sphere and all portions may be stitched together to represent the overall sphere by determining only the respective parameters mentioned above, i.e. piston, tilt and lateral translation.
While the method of stitching interferometry is useful in measuring large spherical optical surfaces by using an interferometer optics of a lower diameter, applications of stitching interferometry to testing of large aspherical surfaces by using null lens systems or compensators having a low diameter did not provide satisfactory results in the past.